The present invention relates to a receiving device and a communication system for adaptively performing frequency allocation in a multicarrier transmission system.
Over the recent years, an OFDM (Orthogonal Frequency Division Multiplexing) system has been adopted for transmission methods of a variety of communication systems, and has realized high-speed data communications at high frequency efficiency. The OFDM system is a system of dividing transmission data into plural pieces of data, mapping the divided pieces of transmission data to a plurality of orthogonal carrier waves (subcarriers), and transmitting the data in parallel on a frequency-axis.
In the individual receiving devices in the communication system that utilizes this type of multicarrier transmission system, frequency bands affected by frequency selective fading differ due to differences of receiving environments thereof. Such being the case, there is a proposed technique for increasing a cell throughput on the whole of the communication system by such a scheme that a transmission-side device performs scheduling for allocating the individual receiving devices serving as communication partner devices to the frequency bands (subcarriers) that are hard to undergo fading.
The frequency scheduling utilized in the multicarrier transmission system such as OFDM will hereinafter be described with reference to FIGS. 10 and 11. FIG. 10 is a diagram showing an example of frequency selective fading, and FIG. 11 is a diagram showing an example of the frequency scheduling. To be specific, FIG. 10 illustrates how the frequency selective fading is in a time block ranging from a timing 2T to a timing 3T in FIG. 11.
The example illustrated in FIG. 10 shows that the signal received by the receiving device of a user 1 has a highest Signal-to-Noise Ratio (which will hereinafter be abbreviated to SNR) in a subcarrier block (frequency block) f1 ranging from the timing 2T to the timing 3T, while the signal received by the receiving device of a user 2 has the highest SNR in a subcarrier block f4.
In this case, the scheduling is done in the transmitting device so as to allocate, as shown in FIG. 11, the subcarrier block f1 having a highly acceptable SNR at the timings 2T to 3T to transmission data for the user 1 and the subcarrier block f4 to the transmission data for the user 2.
Thus, for performing the proper frequency scheduling in the transmitting device, the transmitting device is required to accurately know a reception environment of the reception SNR etc in each of the receiving devices. A technique that each receiving device feeds the self reception environment back to the transmitting device, is adopted from this necessity.
In the case of the communication system in which the single transmitting device communicates with a plurality of receiving device serving as communication partner devices, an operation that all the receiving devices feed the reception SNRs related to all the subcarrier blocks back to the transmitting device, results in a high increase in quantity of consumption of resources of a feedback link. This causes a problem that the frequency efficiency decreases, and resultantly a data transmission throughput on the feedback link declines.
Proposed for solving this problem is a method of reducing a feedback information quantity in such a way that each receiving device feeds back to the transmitting device only positions of N-pieces of subcarrier blocks in the sequence from the subcarrier block having a highest average SNR down to the lowest in the respective received subcarrier blocks (refer to the following Non-Patent document 1). FIG. 12 is a diagram showing a concept of how feedback information is generated by the conventional art described above.
FIG. 12 shows an example of dividing a communication frequency bandwidth into 18-pieces of subcarrier blocks. The receiving device feeds back five high-order average SNRs (#n1 through #n5 shown in FIG. 12) in the average SNRs of the respective subcarrier blocks related to the frequency selective fading. At this time, the receiving device feeds back, for example, the SNRs (SNR(n1), SNR(n2), SNR(n3), SNR(n4), SNR(n5)) of the respective subcarrier blocks.
[Non-Patent document 1] Zhong-Hai Han Yong-Hwan Lee, [opportunistic scheduling with partial channel information in OFDMA/FDD systems], VTC2004-Fall. 2004 IEEE 60th Volume 1, 26-29 Sep. 2004, P 511-514 Vol. 1
The conventional art described above involves feeding back the N-pieces of SNRs in the sequence from the maximum SNR down to the minimum SNR, however, realization of the precise frequency scheduling entails increasing the SNR count (N) for the feedback to some extent, and there is still a problem that the feedback information quantity can not be reduced sufficiently.